1. Field of the Invention
The present invention relates to an internal combustion engine control device for determining properties of fuel according to a change of combustion state, which is calculated on the basis of ion current generated by combustion of a fuel air mixture within a cylinder of the engine after an internal combustion engine is started.
2. Background Art
In a usual gasoline engine, the so-called inlet port fuel injection in which an injector is arranged in an intake port to inject fuel toward an intake valve, is used. In the inlet port fuel injection, substantial portion of injected fuel is directly sucked in a combustion chamber and the remaining fuel is temporarily stuck to the intake valve and/or the intake port and then evaporated and sucked in the combustion chamber. As a result, the amount of fuel sucked in the combustion chamber is changed depending upon evaporation rate of the fuel and the amount of fuel attributing to combustion becomes different even when the amount of injected fuel is constant.
Evaporation rate of fuel is substantially influenced by the property of fuel and the temperature of an intake port portion. Property of commercial gasoline varies in seasons and by gasoline makers. Evaporation rates of low evaporated fuel, such as Summer Gasoline, which is thought as having bad vaporizing rate, and high evaporated fuel, such as Winter Gasoline, which has standard vaporizing rate, with respect to temperature will be described with reference to FIG. 2, in which abscissa shows recovered temperature (fuel temperature) and ordinate shows vaporizing rate (distillated amount). From FIG. 2, it is clear that, although there is no difference in vaporizing rate when distillated temperature (fuel temperature) is a predetermined temperature or higher, vaporizing rate of low evaporated fuel is bad when temperature is low. The vaporizing rates of these fuels tend to be lowered with the lowering of temperature.
When low evaporated fuel is used in a bad vaporizing rate situation such as, for example, low temperature in the vicinity of the intake port, an amount of fuel stuck to the vicinity of the intake port is increased compared with high evaporated fuel with respect to the amount of fuel injected by the injector and an amount of fuel sucked to the combustion chamber, that is, fuel attributing to combustion, is reduced, so that the air fuel ratio becomes lean and combustion is degraded. As a result, idle variation may occur and, in the worst case, stalling of engine may occur. Therefore, the amount of fuel injection is set to a large value so that idle variation or engine stall do not occur in starting a cold internal combustion engine, in which influence of kind of fuel is large, even when low evaporated fuel having low vaporizing rate is used. That is, the problem is solved by increasing the amount of fuel attributing to combustion by increasing the amount of fuel injection.
When high evaporated fuel having standard vaporizing rate is used in the situation in which the amount of fuel injection is set for low evaporated fuel, the amount of fuel sucked in the combustion chamber is larger than that when low evaporated fuel is used. Therefore, the amount of fuel attributing to combustion becomes too much, resulting in that the amount of unburned hydrocarbon exhaust gas is increased.
For example, JP-A-11-336650 discloses a technique in which, when low evaporated fuel is used in cold first idle state, the characteristics of fuel and the state of combustion are detected by detecting reduction of ion current generated when air fuel ratio becomes lean and engine rotation speed is lowered and the amount of fuel injection is corrected correspondingly to the characteristics of fuel and the state of combustion. That is, the combustion state is determined on the basis of the number of times when the amount of ion current generated by fuel combustion in a combustion chamber becomes a reference value or less and the appropriate amount of fuel is obtained by correcting the amount of fuel to the amount of fuel injection corresponding to the combustion state.